1. Field of the Invention
This invention relates to a progressive cavity pump, commonly known as a Moineau type pump.
2. Description of the Prior Art
Progressive cavity pumps azre used in numerous applications because of various characteristics pecular to this type of pump. This type of pump is capable of operating under adverse conditions, for example, where the liquid being pumped contains abrasive particles such as sand. The pump can be made of a very small diameter so that it can be used in spaces, such as a small borehole, where other pumps could not be used. Known designs of progressive cavity pumps are of a high speed and low pressure design.
In stituations where higher than usual output pressures are required, there has been a tendency to increase the interference between the rotor member and stator member. Only a small increase in pressure can be achieved by increasing the interference between the resilient and rigid members, before the loss in efficiency due to frictional resistance becomes too great. Moreover, too great of an interference may require a starting torque which is larger than can be supplied with an electric motor of an acceptable size.
In drill stem testing, it is common practice to isolate a section of a borehole by locating a pair of spaced packers in the annular space around the stem and injecting drilling mud at a high pressure into the packers so as to expand them and thereby seal off the volume between the packers. Although a pump of the progressive cavity type is ideally suited for use within a borehole because of its shape and ability to pump a drilling mud, pressures of 1500 to 2500 p.s.i. are required. With certain designs of the progressive cavity type pump, it is known that the output pressure can be increased by increasing the length of the pump. However, to increase the pressure to the range indicated above, it has been calculated that for a pump having about a 3" diameter, the length would have to be increased to about 16'. It is questionable that a rotor of this length could be satisfactorily driven, and moreover, a pump of this length would not be practical in the above-described drill stem testing application.